John McCain shakes hands with a Libyan rebel during his tour of Benghazi. The Republican senator is the most senior US official to visit to the city – and has strongly backed US intervention. Photograph: Marwan Naamani/AFP/Getty Images

Weeks before the Republican-controlled House voted to rebuke Barack Obama's military campaign in Libya, six GOP senators sent the president a letter. The 60-day period where the War Powers Act allows the president to have troops in battle without congressional authorisation was coming to a close. The senators asked Obama if he intended to seek congressional approval or terminate US involvement in the conflict.

The president did neither. As the half-dozen Republicans pointed out in their letter, the Libya intervention had been conducted without heeding the letter of the law from the beginning. "That action," they wrote, "was taken without regard to or compliance with the requirement of section 2(c) of the War Powers Resolution." There was no declaration of war, no congressional authorisation of force, and no national emergency created by an attack on the United States.

Oddly, half the Republican senators who signed the letter were serving when George W Bush was president. None of them was particularly vocal about the legal limits of presidential war powers back then. Senator John Cornyn (Texas) is a signatory who had harsh words for a nonbinding resolution opposing President Bush's surge in Iraq. "To offer nonbinding resolutions which encourage our enemies and undermine our allies and deflate the morale of our troops is, to me, the worst of all possible worlds," Cornyn said at the time.

Partisan inconsistency on war and executive power isn't limited to Republicans. When the Libya operation began, Senator Rand Paul (Republican, Kentucky) introduced the following sense of the Senate resolution:

"The president does not have power under the constitution to unilaterally authorise a military attack in a situation that does not involve stopping an actual or imminent threat to the nation."

The origin of that language? It's a quote from candidate Barack Obama in 2007.

Debating this resolution, Senator Paul argued that President Bush had at least approached Congress before going to war in Afghanistan and Iraq. Senator Dick Durbin (Democrat, Illinois) fired back the Bush actually "broke precedent" by seeking congressional consent. That would be the same Durbin who, as a member of the Senate intelligence committee, pushed back against the Bush administration's request for congressional authorising the Iraq war, demanded "dramatic changes" to the war resolution, and ultimately voted against going into Iraq.

This is nothing new. When President Bill Clinton intervened militarily in Kosovo, Representative Tom DeLay (Republican, Texas) – a member of the House GOP leadership – protested vigorously. "I rise today to state that no defence funds should be used for ground forces in Kosovo unless authorised by Congress," he said. DeLay's criticisms didn't stop when the airstrikes began. "I cannot support a failed foreign policy," he remarked in 1999. "History teaches us that it is often easier to make war than peace."

But when President Bush went to war in Iraq, DeLay accused antiwar Democrats of "trying to have it both ways". "They start every comment with 'I certainly support the troops,' and then go denigrate why they're there," DeLay said in 2003. "That's not supporting the troops, because you are telling that soldier directly he's risking his life for something that's wrong, and that has consequences."

When Democratic congressional leaders pronounced Bush's Iraq policy a failure, as he once described Clinton's Kosovo adventure, DeLay told a newspaper it was "very, very close to treason". "We have people dying," he said. "Not just our soldiers, but innocent citizens dying in Iraq and Afghanistan at the hands of these evil people, and you have your elected leaders making these kinds of statements that embolden the enemy."

This is a long bipartisan tradition. Under President Richard Nixon, Senator Edward Kennedy (Democrat, Massachusetts) was an outspoken opponent of the Vietnam war. But when he brother John F Kennedy was in office, he said of Vietnam doves, ""I wish they had raised their voices against Viet Cong terrorism, against Viet Cong murder, kidnapping, and political assassination." The late senator argued, "Any facilities in North Vietnam strengthening the Viet Cong should be bombed."

There are some exceptions to the rule. Representatives Ron Paul, a Texas Republican, and Dennis Kucinich, an Ohio Democrat, have been consistently antiwar no matter who is in the White House. Senators John McCain, an Arizona Republican, and John Kerry, Massachusetts Democrat, have been backing military interventions from the Balkans to Libya, no matter which party does the bombing.

In the 1990s, leading congressional Republicans sued the Clinton administration over military action in Kosovo. But the party for the most part backed expansive executive powers during wartime under Bush. With the Democrats again in control of the executive branch, 87 House Republicans voted with Kucinich to force a withdrawal from Libya. Similarly, some Democrats who denounced Guantánamo Bay, wiretapping and targeted assassinations during the Bush administration have been more muted in their criticisms of these policies under Obama.

It's a cliché that truth is the first casualty of war. Partisan consistency runs a close second.